The Vr/60 strain of measles virus produces a characteristic cytopathology in vero cells but infectious cell-free virus is not formed. When Vr/60 chronically infected cells were incubated with lymphocytes from normal donors or SSPE patients a marked decrease in plaque formation was noted. Lymphocyte from individuals with immune deficiencies (a gammaglobulinemia, Sezzari syndrome and IgA deficiency) failed to reduce plaque counts. It has been suggested that this test may be a measure of measles virus-specific cellular immunity. If this is true, it can be concluded that SSPE patients have normal cellular immunity to measles virus. However, this conclusion cannot be drawn unequivocally because the mechanism of plaque reduction in this test has not been elucidated.